AC-to-AC power converters are commonly used in a variety of applications in which it is desirable for an input AC power (e.g., three-phase power provided via power lines) to be modified to a different, output AC power having different voltage, current or other characteristics. For example, AC-to-AC power converters can be used as motor controllers that govern the torque, speed, or other operations of the motors used in industrial or other applications.
AC drives are one type of AC-to-AC power converter that are frequently employed. AC drives typically employ one or more (e.g., in the case of three-phase input power, typically six) controlled switching devices such as Insulated Gate Bipolar Transistors (IGBTs), which are controlled to modify the characteristics of the input AC power to achieve the desired output AC power. By controlling the operation of the controlled switching devices through the use of a sophisticated controller, AC drives allow for a high degree of control over the power conversion process and the ultimate output AC power that is provided. In particular, AC drives typically allow for the modification of the voltage, current, frequency and phase of incoming power and thus are capable of providing outgoing power that has significantly different characteristics than the incoming power.
Despite the high degree of control afforded by AC drives, AC drives are somewhat disadvantageous in that the drives are complicated to control and consequently require the use of sophisticated controllers. Additionally, AC drives are somewhat disadvantageous in that the controlled switching devices (e.g., power semiconductor devices) continually generate significant amounts of heat while the AC drives are operating. Consequently, not only are AC drives somewhat inefficient in terms of the power conversion they provide, but also such drives require specialized heat sinking devices that allow for continual, substantially uninterrupted conduction and/or convection of the generated heat away from the controlled switching devices to avoid an unacceptable heat buildup.
Another type of AC-to-AC power converter is the reduced voltage starter. While reduced voltage starters, like AC drives, employ one or more controlled switching devices, reduced voltage starters generally provide less control over the power conversion process and ultimate output AC power than AC drives. For example, while able to modify the voltage and current of incoming power, reduced voltage starters are unable to modify the frequency or phase of incoming power. Consequently, reduced voltage starters generally are simpler to control and less expensive than AC drives.
A relatively recent advance in reduced voltage starter technology involved the development of the reduced voltage starter with bypass. It was recognized that, in reduced voltage starter applications, power control is only required during a relatively short transitional (or transient) time period between the commencement of providing power to a load and a time at which steady-state operation of the load is attained. This is often the case, for example, with respect to the delivery of power to motors used in pumping, conveyor belts, and elevator and escalator applications. Motors typically require a high degree of current during startup to produce high torque, which must be controlled to avoid unexpected results. Then, once the device attains a standard operating speed, the power demanded by the motor is maintained at a fairly constant, steady-state level, requiring less control.
A reduced voltage starter with bypass takes advantage of the fact that, in certain applications, control over the power conversion process need only be provided during the transitional mode of operation. Such reduced voltage starters include bypass relays/contacts that can be opened or closed and, when closed, shunt together the input and output terminals of the starters to one another (e.g., couple or short-circuit the power line and load terminals of the starter directly to one another) and thereby bypass or shunt out the controlled switching devices. When a reduced voltage starter with bypass is employed, the bypass relays are open during the transitional mode of operation so that the controlled switching devices can be employed to achieve the desired output AC power. Then, when the steady-state mode of operation has been attained, the bypass relays are closed so that the input AC power is directly provided as the output AC power to the load.
Because reduced voltage starters with bypass only utilize the controlled switching devices during the transitional mode of operation, the reduced voltage starters with bypass give off much less heat than both reduced voltage starters without bypass and AC drives, in which the controlled switching devices remain operational at all times whenever power is being delivered to the load. Thus, reduced voltage starters with bypass, though limited in terms of the types of applications (loads) with which they can be employed vis-à-vis AC drives in terms of the level of control that they are capable of providing, are at the same time both easier to control than AC drives and more efficient than both AC drives and reduced voltage starters without bypass.
Further, because reduced voltage starters with bypass only dissipate substantial amounts of power during the transitional mode of operation, and because the periods in which the starters are in their transitional mode are typically separated by long periods of steady-state operation in which there is relatively little heat dissipation (due to the operation of just the bypass relays), reduced voltage starters with bypass typically do not rely upon the same types of heat sinks as are commonly utilized in reduced voltage starters without bypass or AC drives. Instead of using specialized, large heat sinks designed to transport heat away from the controlled switching devices by conduction and then convection, reduced voltage starters with bypass rather employ large thermal masses (e.g., large blocks of copper) having enough heat capacity to absorb heat during the transitional mode of operation and then slowly dissipate that stored heat during the steady-state mode of operation, thereby resetting the thermal masses for the next transitional mode in the application.
Regardless of the type of AC-to-AC power conversion device that is employed, it is desirable in most if not all operating environments that such power conversion devices be minimized in their physical dimensions so that they can easily be installed/implemented in conjunction with other system components. Commonly, power conversion devices are installed on panels. When installed on such panels, the reduction of certain physical size characteristics of the power conversion devices becomes more important than the reduction of others. Namely, it becomes particularly desirable for the width and, to a lesser extent, the length of a power conversion device (as measured parallel to the panel's surface) to be minimized so that several power conversion devices or other devices can all be attached perpendicularly to the same panel, side by side, while the depth of power conversion devices (in terms of the distance that the device extends outward away from the panel) is of the least concern.
Despite this need for more compact AC-to-AC power conversion devices, existing AC-to-AC power conversion devices often remain fairly large. In particular with respect to AC drives and reduced voltage starters without bypass, where large amounts of heat are continuously generated, large heat sinks are typically used to allow for conduction and then convection of the generated heat away from the devices. In certain embodiments, fans, liquid cooling systems or other specialized components are also included for this purpose.
Further, reduced voltage starters in particular (in contrast to some AC drives) tend to be large insofar as they employ single-phase semiconductor-based AC switches in order to guarantee electrical isolation between different phases. That is, reduced voltage starters that are intended to convert multiple phases of power employ multiple physically-discrete switches, each of which includes its own dedicated housing/casing for containing the controlled switching devices used to convert a single respective phase of power and a corresponding thermal mass/heat sink for dissipating the heat generated by those particular controlled switching devices. For example, if three-phase input AC power is being converted by a reduced voltage starter into three-phase output AC power, three individual, physically-discrete AC switches are employed within the reduced voltage starter, with each switch handling one phase.
Additionally (also in contrast to some AC drives), reduced voltage starters typically employ large, discrete circuit components as their controlled switching devices, which typically can include power thyristors/silicon-controlled rectifiers (SCRs). Further, the power thyristors or other control switching devices typically are sandwiched between the thermal masses of the reduced voltage starters in the form of large, off-the shelf press pack structures. Thus, the large space occupied by reduced voltage starters when used for multi-phase applications is a result of both the fact that reduced voltage starters need multiple power modules to handle the multiple phases, and because each power module in itself is relatively large due to its individual housing and due to the large components that must be contained within that housing.
For these reasons, a need still exists for a new AC-to-AC power conversion device that is smaller than existing power conversion devices that provide comparable power conversion capabilities. In particular, given the special advantages provided by reduced voltage starters with bypass in comparison with AC drives and reduced voltage starters without bypass—namely, that reduced voltage starters with bypass are less complex and costly than AC drives and are more efficient than reduced voltage starters without bypass—a need still exists for an improved multi-phase reduced voltage starter with bypass that in particular is smaller than existing multi-phase reduced voltage starters with bypass.